Hitherto, a piezoelectric body, which is composed of materials such as inorganic ceramics or an organic polymer, has been suitably used for a sensor, an actuator, an ultrasound transducer, and the like.
As the inorganic ceramics, there are well known a PbZrO3/PbTiO3 solid solution (PZT), a Pb(Mg1/3Nb2/3)O3/PbTiO3 solid solution (PMN-PT), a Pb(Zn1/3Nb2/3)O3/PbTiO3 solid solution (PZN-PT), and the like. A piezoelectric constant d33 characterizing a piezoelectric body is an index converting force to electric charge. The typical d33 values of the above-described ceramics are 550, 2820 and 2000 (the unit is pC/N), respectively, which are considerably higher as compared with those of polymers described later.
As a representative example of the polymers, there is mentioned vinylidene fluoride-trifluoroethylene copolymer (P(VDF-3FE)) whose d33 is about 17. In order to improve this property in the polymers, there have been hitherto studied organic polymer materials such as a kneaded mixture of P(VDF-3 FE)) with polyurethane or silicone (U.S. Pat. No. 6,689,288), a kneaded mixture of polyvinylidene fluoride with nylon (US patent Publication No. 2002/0166620), polybutadiene, a poly(N,N-methylenebisacrylamide-styrene) copolymer (Japanese Patent O.P.I. Publication No. 2006-049418), a high magnetic field application molded product of poly(γ-benzyl-L-glutamate) (Japanese Patent O.P.I. Publication No. 2005-217111), a polyurea obtained by vapor deposition polymerization of methane diisocyanate and diaminofluorene (Atsushi Kubono, Masashi Mural and Shigeru Tasaka, “High Piezoelectric Activity in Nonpoled Thin Films Prepared by Vapor Deposition Polymerization”, Japanese Journal of Applied Physics, Japan, The Japan Society of Applied Physics, Jul. 11, 2008, Vol. 47, 7, p. 5553-5557), an electret in which air bubbles are incorporated in a tetrafluoroethylene-hexafluoropropylene copolymer (Japanese Patent O.P.I. Publication No. 2007-231077) and the like; and materials such as a PZT-siloxane-poly(meth)acrylate composite (Japanese Patent O.P.I. Publication No. 2002-185054), a composite of polylactic acid with calcium phosphate or montmorillonite (Japanese Patent O.P.I. Publication No. 2005-213376) and the like. However, a satisfactory performance has not yet been obtained.
In recent years, as a property required in addition to D33 described above when a piezoelectric body is applied to a transducer or a sensor, there is a frequency bandwidth in which the piezoelectricity is developed, and it is demanded that the piezoelectric body can be utilized in a broad frequency bandwidth.
When a piezoelectric body having a broad bandwidth is used, for example, in a medical ultrasound diagnostic system, a single piezoelectric body enables both transmission of a low frequency ultrasound capable of being transmitted to deeper diagnosis domain and reception of high spatial resolution information in which high order harmonic components are superposed by non-linear propagation. Hitherto, no piezoelectric body with a broad band property has been found which can realize such a performance, and a high sensitive piezoelectric body for transmission and a high sensitive piezoelectric body for reception have been separately employed according to the respective frequency band.
As the index of the bandwidth (−6 dB bandwidth), there is a ratio of a frequency providing the highest output and the difference between the maximum frequency and the minimum frequency where the highest output is reduced to half (−6 dB attenuation).
The ratio in the ceramics as described above is in the range of from 10 to 70%, and the ratio in the known polymer materials is in the range of from 80 to 400%.
As is apparent from the aforementioned, there is a contradictory relationship between the bandwidth and d33, and it is extremely difficult to enhance both properties simultaneously. When a material in which both properties are simultaneously enhanced is used in a transducer, a medical ultrasound diagnostic system and a nondestructive ultrasound test system, each employing the transducer, provide diagnosis with high precision and detection with high accuracy, respectively.
In order to solve this problem, composites of PZT and polymer materials have been developed, and among these, a 1-3 composite is representative in which the longitudinal of the PZT prism is oriented in the vibration direction (see Non-Patent Document 1). The d33 and bandwidth of this composite is in the range of from 50 to 200 pC/N and in the range of from 50 to 150%, respectively, although the d33 and bandwidth depend mainly on the configuration of the PZT prism (see Non-Patent Document 2).
Proposed is an attempt which employs, instead of PZT, a single crystal (d33=1200 cP/N) of [Pb(Mg1/3)(Nb2/3)O3]0.68[PbTiO3]0.32 which enhances d33 (see Patent Document 1). However, the d33 of the resulting 1-3 composite is 120 pC/N, and the property of the single crystal cannot be effectively utilized.
A composite material obtained by calcining, at a low temperature, fullurene and PZT prepared according to a sol-gel method is known (see Patent Document 2). However, the composite material, containing fullurene dispersion particles exceeding 200 nm and having a fullurene content of 10% by volume, is far lower in the piezoelectricity than PZT, and hardly broadens the bandwidth.
As the bandwidth range of the frequency used is broadened, there is high demand to further improve these properties.
Examples, in which halide based perovskite compounds in the form of layers are applied to an electroluminescence element and the like, have been hitherto known (see, for example, Patent Document 3), however, it is unknown that these are used as a piezoelectric body.